As is well known, a thick-film thermal printhead has a heating resistor and an electrode pattern (including a common electrode and individual electrodes) formed by printing and baking a conductive paste. FIG. 11 of the accompanying drawings is a sectional view showing an example of prior art thermal printhead. The illustrated thermal printhead B includes a substrate 100 which is provided, entirely on the upper surface thereof, with a glaze layer 110 for heat retention. A common electrode 120 and a plurality of individual electrodes (not shown) are formed on the glaze layer 110. The thermal printhead B further includes a heating resistor 130 electrically connected to the common electrode 120 and the individual electrodes.
An common electrode auxiliary layer 140 is formed on the common electrode 120 at a portion spaced from the heating resistor 130. The common electrode auxiliary layer 140 is provided for preventing a voltage drop in the common electrode 120.
The thermal printhead B has an overcoat layer 150 for covering the common electrode 120, non-illustrated individual electrodes, the heating resistor 130 and the common electrode auxiliary layer 140. Further, a protective layer 160 which is thinner than the overcoat layer 150 is formed on the overcoat layer 150. The protective layer 160 is formed of a material which is less susceptible to wear and scratches than the material for the overcoat layer 150. With such a structure, the common electrode 120 and other parts are prevented from directly contacting a recording paper S. As shown in FIG. 11, the protective layer 160 is formed not only on the upper surface of the overcoat layer 150 but also continuously on a side surface 100s of the substrate 100.
As shown in FIG. 11, a platen roller C is provided on the thermal printhead B so as to contact the protective layer 160. The platen roller C rotates in the direction of the arrow D1 to transfer the recording paper S in the direction of the arrow D2 in close contact with the protective layer 160. At this time, the recording paper S transferred outwardly by the platen roller C warps downwardly by its own weight. The substrate 100 and the glaze layer 110 are chamfered in such a manner as to correspond to such a warp. As a result, the substrate 100 is formed with a first bevel portion 100a, whereas the glaze layer 110 is formed with a second bevel portion 110a. Accordingly, it is possible to prevent the recording paper S from being caught at a corner of the substrate 100 (or of the glaze layer 110), and therefore, it is possible to transfer the recording paper S smoothly outwardly by the platen roller
While having the advantages described above, the prior art thermal printhead B has the following problems.
First, due to the difference in thermal expansion coefficient between the glaze layer 110 and the protective layer 160, the protective layer 160 in the form of a thin film may break or may be released from the glaze layer 110. Specifically, the protective layer 160 directly covers the glaze layer 110 at a portion of the upper surface and continuously at the inclined portion 110a. When the glaze layer 110 and the protective layer 160 are heated, they thermally expand to different degrees with each other. As a result, stress is concentrated on the ridge 160a of the protective layer 160, resulting in the breakage of the protective layer 160.
Secondly, with the structure of the prior art thermal printhead B, it is impossible to sufficiently urge the recording paper S toward the heating resistor 130, which may cause improper printing. As shown in FIG. 11, the protective layer 160 has a first convex portion 160b (a portion above the heating resistor 130) and a second convex portion 160c (a portion above the common electrode auxiliary layer 140), with both of which convex portions the platen roller C engages. However, because of the existence of the common electrode auxiliary layer 140, the second convex portion 160c is located considerably higher than the first convex portion 160b (See the sign "t" in the drawing). With such a structure, the pressing force by the platen roller C is mostly exerted on the second convex portion 160c, so that the recording paper S is not sufficiently pressed against the first convex portion 160b. As a result, heat from the heating resistor 130 is not sufficiently transmitted to the recording paper S, which may cause printing failure such as unclear printing results.